1. Field of the Invention
The present invention relates to a driving circuit for a light emitting element used in an image display unit, and more particularly to a pixel driving circuit for a display device in which opening ratio of the light emitting element is improved by using a common driving circuit for each pixel, thereby reducing a number of elements in a driving circuit for driving the light emitting element installed inside a panel of a display device.
2. Description of Related Art
An organic electroluminescent (EL) display device performs display by applying a current from a pixel electrode formed per pixel to an organic electroluminescent (EL) device. The organic EL display device can be classified into a passive matrix type display device or an active matrix type display device. The active matrix type display device includes a switching element installed at each pixel inside an organic EL panel 30 and carries out image display in response to a control voltage or current corresponding to image data of the pixel as illustrated in FIG. 1.
FIG. 1 is a block diagram illustrating a conventional active matrix type organic EL display device.
As illustrated in FIG. 1, an active matrix type organic EL display device includes a data driver 10 for outputting image data, a scan driver 20 for outputting selection signals, data lines D_R1, D_G1, D_B1, . . . D_Rn, D_Gn, D_Bn coupled to the data driver 10, and gate lines S1, S2, . . . Sm-1, Sm coupled to the scan driver 20. As shown in FIG. 1, an organic EL panel 30 includes a plurality of pixels 31 that are longitudinally and laterally arranged and coupled to corresponding ones of the data lines and the gate lines, respectively. Each pixel 31 is a combination of red, green and blue unit pixels, and is formed at a corresponding intersection between the gate lines and the data lines in the organic EL panel 30.
Therefore, if image data are received from the data driver 10 and scan signals are received from the scan driver 20, each pixel driving circuit transmits relevant driving signals to a corresponding light emitting element according to the received signals so that each pixel 31 displays respective colors according to the combination of red, green and blue. That is, a conventional pixel includes a driving circuit per each pixel so that the driving circuit is connected to the gate lines and data lines, respectively. Therefore, the pixel displays one pixel data by individually driving each of the unit pixels in response to the received scan signals and data signals.
FIG. 2 is a schematic diagram illustrating a conventional pixel driving circuit.
As illustrated in FIG. 2, the conventional pixel is a combination of red, green and blue unit pixels formed at an intersection between data lines and gate lines, and each unit pixel includes a driving circuit for driving a corresponding one of the EL devices. In other words, each of the driving circuits for driving one of the unit pixels on the same row is connected to a different one of the data lines, but to the same gate line. By way of example, driving circuits located on the same row are connected to only one gate line S1, but are connected to data lines D_R1, D_G1, D_B1, D_R2, D_G2, D_B2 . . . D_Rn, D_Gn and D_Bn, respectively.
A gate of a first thin film transistor M1 is connected to a gate line Scan, and a source of the first thin film transistor M1 is connected to a data line D_R1. Furthermore, a first capacitor C1 is connected between a drain of the first thin film transistor M1 and a first power supply voltage Vdd. A gate of a second thin film transistor M2 is connected between the first capacitor C1 and the drain of the first thin film transistor M1. The first power supply voltage Vdd is connected to a source of the second thin film transistor M2, and an anode of a red EL device R is connected to a drain of the second thin film transistor M2. In addition, a cathode of the red EL device R is connected to a second power supply voltage Vss.
The second power supply voltage Vss is also connected to a cathode of a green EL device G, and a drain of a fourth thin film transistor M4 is connected to an anode of the green EL device G. The first power supply voltage Vdd is connected to a source of the fourth thin film transistor M4, and a drain of the third thin film transistor M3 is connected to a gate of the fourth thin film transistor M4. Further, the gate line Scan is connected to a gate of the third thin film transistor M3, and a data line D_G1 is connected to a source of the third thin film transistor M3. A second capacitor C2 is connected between the gate of the fourth thin film transistor M4 and the first power supply voltage Vdd.
Further, a drain of a sixth thin film transistor M6 is connected to an anode of a blue EL device B, and the second power supply voltage Vss is connected to a cathode of the blue EL device B. The first power supply voltage Vdd is connected to a source of the sixth thin film transistor M6, and a drain of the fifth thin film transistor M5 is connected to a gate of the sixth thin film transistor M6. A third capacitor C3 is connected between the gate of the sixth thin film transistor M6 and the first power supply voltage Vdd. Further, the gate line Scan is connected to a gate of the fifth thin film transistor M5, and a data line D_B1 is connected to a source of the fifth thin film transistor M5. The cathode of the said red, green and blue EL devices are connected to the second power supply voltage Vss.
The first, third and fifth thin film transistors M1, M3 and M5 are turned on in response to a scan signal applied on the gate line Scan through a sequential selection of the gate lines by the scan driver 20. Therefore, image signals applied to the respective data lines D_R1, D_G1, D_B1 by the data driver 10 are inputted into the source side of the thin film transistors M1, M3, M5 and stored in the capacitors C1, C2, C3, respectively. Therefore, second, fourth and sixth thin film transistors M2, M4, M6 are turned on to transfer the first power supply voltage Vdd transferred from the source side and a current corresponding to a square of the difference between the data voltage and the threshold voltage to the respective red, green and blue EL devices so that the red, green and blue EL devices are emitted according to the magnitude of the applied current.
Referring to a driving waveform diagram of FIG. 3, the operation of a conventional organic EL display device described above is further described as follows.
In reference to FIGS. 1 and 3, first, if a scan signal S1 is applied on a first gate line S1, the first gate line S1 is driven, and pixels PR11-PB1n connected to the first gate line S1 are driven.
That is, the switching thin film transistors M1, M3, M5, respectively, of is red, green and blue unit pixels PR11-PR1n, PG11-PG1n, PB11-PB1n connected to the first gate line S1 are driven by the scan signal S1 applied on the first gate line S1. Red, green and blue data signals D1(D_R1-D_Rn), D1(D_G1-D_Gn), D1(D_B1-D_Bn) are simultaneously applied on the gates of the driving thin film transistors M2, M4, M6 of red, green and blue unit pixels, respectively, through the red, green and blue data lines D_R1-D_Rn, D_G1-D_Gn, D_B1-D_Bn that constitute first to nth data lines D1, . . . Dn in response to the driving of the switching thin film transistors M1, M3, M5.
The driving thin film transistors M2, M4, M6 of red, green and blue unit pixels supply driving current corresponding to red, green and blue data signals D1 (D_R1-D_Rn), D1 (D_G1-D_Gn), D1 (D_B1-D_Bn), respectively, that are applied through the red, green and blue data lines D_R1-D_Rn, D_G1-D_Gn, D_B1-D_Bn to the red, green and blue EL devices. Therefore, the EL devices including pixels PR11-PB1n connected to the first gate line S1 are simultaneously driven when the scan signals are applied to the first gate line S1.
In a similar manner, if scan signals for driving a second gate line are applied on a second scan line S2, data signals D2(D_R1-D_Rn), D2(D_G1-D_Gn), D2(D_B1-D_Bn) are applied to pixels PR21-PR2n, PG21-PG2n, PB21-PB2n connected to the second gate line S2 through the red, green and blue data lines D_R1-D_Rn, D_G1-D_Gn, D_B1-D_Bn.
The EL devices including the pixels PR21-PR2n, PG21-PG2n, PB21-PB2n connected to the second gate line S2 are simultaneously driven by driving current corresponding to the data signals D2(D_R1-D_Rn), D2(D_G1-D_Gn), D2(D_B1 -_Bn).
The EL devices including pixels PRm1-PBmn connected to the mth gate line Sm are simultaneously driven in response to red, green and blue data signals Dm(D_R1-D_Rn), Dm(D_G1-D_Gn), Dm(D_B1-D_Bn) applied on the red, green and blue data lines D_R1-D_Rn, D_G1-D_Gn, D_B1-D_Bn when a scan signal Sm is applied to the mth gate line Sm by repeating the foregoing operations.
Therefore, if the scan signals are sequentially applied on the gate lines S1 through Sm, the pixels (PR11-PB1n)-(PRm1-PBmn) connected to the respective gate lines S1-Sm display an image by being driven sequentially during one frame.
However, in an organic EL display device having the structure described above, each pixel includes red, green and blue unit pixels and driving elements (i.e., switching thin film transistor, driving thin film transistor and capacitor) for driving red, green and blue EL devices, for the red, green and blue unit pixels, respectively, are duplicated. Further, data lines and common power supply lines for supplying data signals and the power supply voltage Vdd to each driving element are also duplicated.
Therefore, three data lines and three power supply lines are arranged per pixel, and six transistors (i.e., three switching thin film transistors and three driving thin film transistors) and three capacitors are required for each pixel. Therefore, in the conventional Organic EL device, the circuit structure is complicated as a plurality of wirings and elements are used for each pixel, and an opening ratio of the light emitting elements is limited. Further, the yield also is decreases accordingly during the manufacturing process.
Further, in the conventional Organic EL device, the area of each pixel is reduced as the display device is gradually being made to have a higher precision, and not only is it difficult to arrange many elements on one pixel, but also the opening ratio is reduced accordingly.